1. Field of the Invention
The present invention relates to optical pickups compatible with recording media having different formats, and more particularly, to a compact optical pickup compatible with recording media with different formats, which uses a single light device module having two light beam sources with different light wavelengths.
2. Description of the Related Art
In recent years, there has been a need for an optical pickup capable of recording information on and/or reproducing information from a digital versatile disc-read only memory (DVD-ROM) at high densities that is compatible with the compact disc (CD) family of media. The CD family of media includes the CD, the recordable CD (CD-R), the CD rewritable (CD-RW), the CD interactive (CD-I), and the CD plus graphics (CD+G) compatible media.
The standard thickness of the CD family media is 1.2 mm, whereas the thickness of DVDs has been standardized to 0.6 mm, which takes in consideration of the allowable error in the tilt angle of an optical disc and the numerical aperture (NA) of an objective lens. Accordingly, when recording information on or reproducing information from a CD using an optical pickup designed for DVDs, spherical aberrations occur due to a difference in the thicknesses therebetween. Such spherical aberration cannot provide a light intensity sufficient for recording an information (radio frequency) signal or may deteriorate a reproduction signal from the CD. Also, DVDs and CD families of media utilize different wavelengths of light for reproduction. Specifically, CDs use light having a wavelength of about 780 nm, whereas DVDs use light having a wavelength of about 650 nm. Thus, in order to be compatible with CDs, an optical pickup needs to use a light beam source capable of emitting different wavelengths of light, and capable of focusing optical spots at different focal positions.
Referring to FIG. 1, a conventional compatible optical pickup comprises a first light beam source 21 to emit a light having a wavelength of about 650 nm, and a second light beam source 31 to emit a light having a wavelength of about 780 nm. The first light beam source 21 is appropriate for a relatively thin optical disc 10a, such as a DVD, and the second light beam source 31 is appropriate for a relatively thick optical disc 10b, such as a CD. The thin optical disc 10a and the thick optical disc 10b are generically referred to as an optical disc 10.
Light emitted from the first light beam source 21 is collimated by a first collimator lens 23 to be parallel and incident on a first polarization beam splitter (PBS) 25, and then reflected by the first PBS 25 toward the thin optical disc 10a. After being reflected by the thin optical disc 10a, the reflected light is transmitted through the first PBS 25 and is received by a first photodetector 27. Here, an interference filter 41 that changes the paths of light emitted from the first and second light beam sources 21 and 31, a quarter-wave plate 43, a variable diaphragm 45, and an objective lens 47 to condense light incident are disposed on an optical path between the first PBS 25 and the thin optical disc 10a. 
Light emitted from the second light beam source 31 is collimated by a second collimator lens 33 to be parallel and incident on a second PBS 35, transmitted through a condenser lens 37, and then incident on the interference filter 41. The light is reflected by the interference filter 41 and sequentially passes through the quarter-wave plate 43, the variable diaphragm 45, and the objective lens 47 to form an optical spot on the thick optical disc 10b. 
Light reflected by the thick optical disc 10b is incident on the interference filter 41 through the objective lens 47, the variable diaphragm 45 and the quarter-wave plate 43, and then reflected by the interference filter 41 towards the second PBS 35. The reflected light is reflected by the second PBS 35 and received by a second photodetector 39.
The interference filter 41, which is an optical element that transmits or reflects incident light depending on the wavelength of incident light, transmits the light originating from the first light beam source 21, and reflects the light originating from the second light beam source 31. The variable diaphragm 45 has a variable aperture, and defines the size of the light spot incident on the objective lens 47 such that the light beam enters the region of the objective lens 47 with an NA less than 0.45 or 0.47. The quarter-wave plate 43 is an optical element for changing the polarization of incident light. As each of the light beams from the first and second light beam sources 21 and 31 passes the quarter-wave plate 43 two times, the polarization of the incident beams changes, and then the light beams head toward the first PBS 25 and the second PBS 353, respectively. The objective lens 47 allows light from the first and second light beam sources 21 and 31 to be focused as a light spot on the corresponding recording surfaces of the thin optical disc 10a and the thick optical disc 10b. 
Although the conventional optical pickup having the configuration described above is compatible with a CD-R using two light beam sources, the use of the variable diaphragm, which is manufactured through sophisticated and expensive processes, makes assembling of such an optical pickup complicated and increases the manufacturing cost. In addition, the first and second light beam sources are separately constructed to further complicate the configuration and optical arrangement of the optical pickup.